Provided are a method for obtaining an HNL gene and HNL derived from a millipede other than Chamberlinius hualienensis, and preparing a practically useable amount of HNL; and a method for producing optically active cyanohydrin using this HNL. A method for producing a millipede-derived HNL gene. A method that includes the selection of a gene having a base sequence that encodes a conserved amino acid sequence TAX1DIX2G (SEQ ID NO: 15) or VPNGDKIH (SEQ ID NO: 16) of millipede-derived HNL from genes present in an organism belonging to the Diplopoda. A protein having an amino acid sequence of any of (1)-(3) and having HNL activity. (1) An amino acid sequence listed in any of SEQ ID NOS: 1, 3, 5, 7, 9, 11, 13, 83, 85, 87, or 89; (2) an amino acid sequence having amino acids deleted, substituted, and/or added in an amino acid sequence of (1); or (3) an amino acid sequence having 90% or greater identity to an amino acid sequence of (1). A method for preparing optically active cyanohydrin by causing this millipede-derived HNL to act on a reaction solvent that contains an aldehyde or the like and a substance that generates hydrogen cyanide or the like.